PURPOSE: To control the number of pump units by calculating the number of operating units of water supply pumps according to the relation of the water level of the distributing reservoir in the second water supply system and the flow rate of the supplied water.

CONSTITUTION: A distributing reservoir 12 is formed on the consumption side of the second water supply system 4, and a supplied water flow rate detector 9 and a function generator 10 are arranged. When the flow rate of the first water supply system 3 varies, the flow rate of the second water supply system 4 varies. According to the relation of the output signals of the flow rate detector 9 and a distributing reservoir water level detector 13 and the output signal of the function generator 10, if the upper limit value < the measured value, the number of operating units is added, and if the upper limit value ≥ measured value ≥ the lower limit value, existing operation is continued, and if the lower limit value > the measured value, the number of operating units is reduced, thus the number of operating units of the water supply pumps 2 can be controlled. Accordingly, when the water supply amount of the second water supply system 4 and the water level of the distributing reservoir are within the ranges set, the operation state of the water supply pump is not changed, and the frequency of start can be reduced, and economical and effective operation can be obtained.